This invention relates to the establishment of a clean working environment and, more particularly, to an air handling system for a laminar flow clean enclosure.
In a number of industries, such as semiconductor integrated circuit manufacture, food processing, and the manufacture of medical supplies and pharmaceuticals, it is necessary or highly desirable to work in a dust-free or decontaminated, i.e., clean environment. In such case, the work is performed in a clean room or work station. One commonly used technique described in Whitfield U.S. Pat. Nos. 3,158,457 and 3,273,323, uses a large volume of filtered air under laminar flow conditions to establish a clean air environment. Specifically, a planar, submicron, so-called HEPA filter having a large cross-sectional area extends between two spaced apart side walls of a closed or partially open enclosure, such as a room or a work station. An air blower supplies air to the inlet of the filter via a plenum chamber adjacent to the enclosure under conditions that establish laminar air flow from the outlet of the filter in a direction transverse to the plane of the filter. An approximately uniform exit air velocity is required across the entire cross section of the filter outlet to maintain laminar flow conditions; in the presence of human movement and other normal ambient currents caused by heating and air conditioning, an airflow velocity of 90 feet per minute or better .+-.20% is customarily needed to prevent mixing of the clean air with contaminated ambient air. This technique may be viewed as producing an air piston across the full cross-sectional area of the enclosure that continuously moves through the enclosure.
As a practical matter, there is a limitation on the space that can be devoted to the plenum chamber feeding the inlet of the filter. The blower outlet has a much smaller cross-sectional area than the filter and emits air at a much larger velocity than the desired exit velocity from the filter. This creates a problem in reducing the high velocity of the air at the blower outlet before it reaches the filter inlet. If proper steps are not taken to so reduce the air velocity, uniform exit velocity across the filter outlet cannot be maintained and therefore turbulence will result. Present laminar flow air handling systems attempt to convert the entire velocity head at the blower outlet to static pressure before the air reaches the filter inlet by use of baffles and screens, which dissipate a substantial part of the energy imparted to the air by the blower. The static pressure is then partially reconverted to velocity head to provide the desired air velocity through the filter. This process results in substantial energy inefficiencies.